1. Field of the Invention
The present invention relates to a positive resist composition, in particular, a chemically amplified positive resist composition suitable for an ion implantation process; and to a patterning process using this positive resist composition.
2. Description of the Related Art
In recent years, as LSI progresses toward higher integration and further acceleration in speed, miniaturization of a pattern rule is required. In the light exposure used as a general technology nowadays, resolution inherent to wavelength of a light source is approaching to its limit. In 1980s, a g-line (436 nm) or an i-line (365 nm) of a mercury lamp was used as an exposure light at the time of forming a resist pattern. As a mean for further miniaturization, shifting to a shorter wavelength of an exposing light was assumed to be effective. As a result, in a mass production process after DRAM (Dynamic Random Access Memory) with 64-megabits (0.25 μm or less of a processing dimension) in 1990s, a KrF excimer laser (248 nm), a shorter wavelength than an i-line (365 nm), was used in place of an i-line as an exposure light source. However, in production of DRAM with an integration of 256 M and higher than 1 G which require further miniaturized process technologies (process dimension of 0.2 μm or less), a light source with a further short wavelength is required, and thus, a photo lithography using an ArF excimer laser (193 nm) has been seriously investigated since about a decade ago. At first, an ArF lithography was planned to be applied to a device-manufacturing starting from a 180-nm node device, but a KrF excimer laser lithography lived long to a mass production of a 130-nm node device, and thus a full-fledged application of an ArF lithography started from a 90-nm node. Further, a study of a 65-nm node device by combining with a lens having an increased NA till 0.9 is now underway. Further shortening of wavelength of an exposure light is progressing towards the next 45-nm node device, and for that an F2 lithography with a 157-nm wavelength became a candidate. However, there are many problems in an F2 lithography; an increase in cost of a scanner due to the use of a large quantity of expensive CaF2 single crystals for a projector lens, extremely poor sustainability of a soft pellicle, which leads to a change of an optical system due to introduction of a hard pellicle, a decrease in an etching resistance of a resist film, and the like. Because of these problems, it was proposed to postpone an F2 lithography and to introduce an ArF immersion lithography earlier.
In an ArF immersion lithography, a proposal is made to impregnate water between a projector lens and a wafer. A refractive index of water at 193 nm is 1.44, and therefore a pattern formation is possible even if a lens with a numerical aperture (NA) of more than 1.0 is used, and moreover, theoretically NA may be increased to near 1.44. In the beginning, deterioration of a resolution and a shift of a focus due to a change of refractive index associated with a change of water temperature were pointed out. However, the problems associated with the change in the refractive index have been solved by controlling the water temperature within 1/100° C. In addition, it was also confirmed that the effect of heat generation from a resist film by light exposure was almost insignificant. As to the concern of a pattern transcription of microbubbles in water, it was also confirmed that formation of bubbles from a resist film by light exposure was insignificant if water is fully degassed. In the early period of an immersion lithography in 1980s, a proposal was made to immerse an entire stage into water. However, a partial fill method having a nozzle of water supply and of drainage in which water is introduced only between a projector lens and a wafer in order to meet the movement of a high-speed scanner was adopted. By an immersion method using water, designing of a lens with NA of 1 or higher became theoretically possible. However, there appeared a problem in it that a lens dimension in an optical system based on a conventional refractive index system becomes extraordinary large thereby leading to distortion of a lens due to its own weight. A proposal was made to design a catadioptric optical system for a more compact lens, which accelerated a speed in designing a lens having NA of 1.0 or more. Now, mass-production of a device with a 45-nm node is underway by combining a lens having NA of 1.35 with a super resolution technology.
To form a p-well and an n-well of a CMOS device, implantation of an ion is sometimes performed by using a KrF resist pattern as a mask; however, use of an ArF resist pattern has been investigated as progress of miniaturization continues. To perform ion implantation, surface of a substrate in a space portion of a resist pattern needs to be exposed. This is because, if an anti-reflection coating (BARC) layer exists under a resist film, an ion is trapped in the BARC layer. However, if patterning of a resist film is performed without BARO, a standing wave due to substrate reflection is generated thereby causing a significant concavity and convexity on a side wall of the resist pattern after development. To smooth this irregularity caused by the standing wave, use of an acid generator (PAG) to generate an acid having small molecular weight so as to increase acid diffusion thereby readily diffusible, or application of high temperature PEB is considered to be effective. In the size of 200 to 300 nm to which the ion implantation resist pattern of KrF exposure can resolve, there was no deterioration of a resolution due to increase of acid diffusion; but in the size of 200 nm or less to which the ion implantation resist pattern of ArF exposure can resolve, there appears undesirable effects such as deterioration of a resolution due to acid diffusion and increase of a proximity bias.
Use of a resist which contains a dye to avoid generation of a standing wave by letting a photoresist itself have absorption is the most classical way; and investigation thereof has been carried out since the novolak resist of an i-line and a g-line. As to the absorbing ingredient used in the ArF exposure, studies are made on introduction of a benzene ring into a polymer and on an additive which contains a benzene ring. However, the standing wave cannot be completely avoided by the absorbing ingredient; and further, if absorption is made larger, there appears a problem of a tapered trapezoidal form in a cross section of the resist pattern even though the standing wave can be reduced.
A naphthalene ring has a higher etching resistance than a benzene ring; and thus, application thereof to a resist polymer has been attempted (Patent document 1 and 2). Especially, a naphthalene ring or an acenaphthene having a hydroxyl group has a merit of improved adhesion to a substrate as compared with an adhesion group having only a lactone ring.
An ion implantation resist composition to form a pattern by a KrF exposure, wherein a blend of a cresol novolak with a methacrylate having an acid-labile group is used, is proposed (Patent document 3). In this case, there is an economical merit because a cheap cresol novolak resin can be used; but, this cannot be applied to an ArF exposure because the cresol novolak has a strong absorption.
An ion implantation resist composition added with a hydroxy naphthalene or a dihydroxy naphthalene, the both being substituted or unsubstituted with an acid-labile group, is proposed (Patent document 4). Implantation characteristics onto a non-planar substrate can be improved by addition of a monomer component thereinto. However, a naphthalene is prone to sublime so that the naphthalene component contained therein may evaporate during baking; and thus, there is a risk of causing a problem of attachment thereof to the ceiling of a hot plate.
An ion implantation resist composition containing a naphthalene ring and an acenaphthene is proposed (Patent document 5). A polymer which is obtained by copolymerization of a monomer containing a naphthalene, an acenaphthylene, a monomer containing an acid-labile group, and a monomer containing an adhesive group of a lactone is used as a base polymer therein. Here, if the naphthalene has a hydroxyl group, adhesion with a substrate improves; but, there is no improvement in implantation characteristics.
A resist composition for a thick film which is a blend of a t-butyl(meth)acrylate with a novolak resin is proposed (Patent document 6). Here, a novolak resin using a dihydroxy naphthalene is mentioned as the novolak resin. However, if this is used for ion implantation, an acid-labile group of the t-butyl group is poor in dissolution-preventing power thereby leading to poor lithography; and thus, there is a problem of poor masking function at the time of ion implantation.
A KrF resist and an EB resist added with a phenol phthalein, a phenol red, a naphtholphthalein, or a fluorescein as a dissolution-preventing agent, wherein a hydrogen atom of a hydroxyl group in them is substituted with an acid-labile group, are proposed (Patent document 7). It is described therein that a phenol phthalein, a phenol red, a naphtholphthalein, and a fluorescein generate a carboxyl group or a sulfo group in an aqueous alkaline solution thereby accelerating dissolution rate of an exposed area along with de-protection of the acid.